Frequency hopping system for intermittent transmission

ABSTRACT

A radio transmission system including many radio transmitters using frequency hopping carriers to intermittently transmit very short messages indicative of status of sensors associated with the transmitters. In operation, a time interval generator included in a transmitter generates pulses activating the transmitter at time intervals according to a predetermined algorithm. When activated, the transmitter transmits a message at one or several different frequencies. The frequencies are changed according to a predetermined algorithm and preferably differ for each subsequent transmission. Alternatively, when an abnormal sensor status is detected, the transmitter transmits repeated messages at a plurality of predetermined alarm frequencies for a predetermined time regardless of the time interval generator. The system also includes one or more receivers containing a plurality of memory registers to hold digital data indicative of (a) the time and (b) the frequency of the next transmission occurrence independently for each transmitter. The registers are programmed separately for each transmitter based on the time, frequency, and the content of the received messages.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to radio transmission systems in which aplurality of transmitters intermittently transmit short messagesindicative of status of sensors associated with the transmitters and toa method of synchronization suitable for using frequency hopping carrierin such systems.

2. Description of the Prior Art

In certain types of radio transmitter systems there exist manytransmitters that periodically transmit very short messages to one ormore receivers. One example of such systems is burglary and fire alarmsystems. In these systems, many transmitters located at different placesin a building transmit messages indicative of the status of monitoringsensors to a receiver that collects the data from the sensors. Normally,the transmitters transmit supervisory status messages that are as shortas feasible and the period between the transmissions is as long asfeasible in order to minimize the average current drain as thetransmitters in these systems are typically battery operated. Inaddition, short and infrequent transmissions lower the probability thatthe data is lost due to collisions which occur when two or moretransmitters transmit at the same time. However, when an alarm conditionoccurs, a transmitter transmits immediately in order to convey the alarmmessage with little delay.

Typically, such systems transmit data at a single frequency. Thus, theyare susceptible to interference and signal loss due to phenomena knownas multipath fading. Consequently, the reliability of such systems iscompromised or conversely, the transmitted power has to be increased toovercome the fading which results in larger power drain and shorterbattery life. Besides, there usually are regulatory limits that restrictsuch transmitter power and thus limit the possible compensation by sheerincrease of power. Since the multipath effect is highly sensitive to thefrequency of the transmitted carrier, the frequency hopping spreadspectrum technique has a potential to eliminate these drawbacks.However, frequency hopping systems require long acquisition time andthey are typically used in two way communication applications in whichall the devices are continuously synchronizing with one master device orwith each other using a variety of synchronization methods as shown insome of the following references. In other cases, to ease thesynchronization problem, there are employed multichannel receivers thatcan simultaneously receive signals at many frequencies by making thereceiver broadband or by using several receivers at the same time.Generally, those solutions suffer from performance degradation or highcost or both which makes them less desirable for low cost applicationsthat require high reliability such as security alarms.

For example according to U.S. Pat. No. 4,843,638 granted to Walters, areceiver local oscillator has a comb spectrum. This effectively makes ita wide band since each of the frequency range down-converted by thespectral components of the local oscillator will fall in the receiverpassband. Consequently, the sensitivity of such receiver will beadversely affected.

In another example according to U.S. Pat. No. 5,428,602 granted toKemppainen, each hopping frequency is monitored by a separate receiver.This is very costly and presently not suitable for low cost systems.

In another example according to U.S. Pat. No. 4,614,945 granted toBrunius et al., a system is described that allows multiple instrumentsto be monitored and data to be simultaneously transmitted by severalradio transponders. However, in order to operate properly, thetransponders have to be energized by a RF signal to begin a transmissionsequence. This necessitates a radio receiver to be included in thetransponder. This makes the system a two-way communications system. Suchsystems are inherently more complex and costly than one-waycommunications systems.

In yet another example according to U.S. Pat. No. 5,659,303 granted toAdair, a transmitting apparatus is described that transmits bursts ofdata continuously at varying time intervals and at varying frequencies.However, the apparatus as described in the preferred embodiment andassociated claims, allows identical hopping pattern to be realized inthe transmitters. A means is provided to offset the starting point ofthe variation sequence for different transmitters depending on thetransmitter ID, so that the hopping sequences in various transmittersare initially offset in respect to each other. However, due tounavoidable reference frequency drifts that are different in varioustransmitters, the sequences may slide in respect to each other.Consequently, it is only a matter of time that the patterns of two ormore transmitters will be aligned thus producing a condition forpersistent collisions of the transmitted data bursts. In addition, Adairdoes not provide for a receiving apparatus or a method that would allowsuch transmitted signals with varying frequencies to be received. In thecase of Adair's invention, the actual sequence used by a transmitter isnot predetermined but instead it may vary with temperature and dependson the transmitter circuit design and manufacturing tolerances,therefore the signal acquisition is made even more difficult.

A serious problem that must be addressed in battery operated systems isto shorten the transmission time as much as possible by making themessage preamble as short as possible in order to conserve the batterypower. Therefore, the synchronization of the receiver with thetransmitters is a difficult task. This problem is exacerbated in somesystems such as security alarms that require some messages to beconveyed to the system immediately without waiting for the scheduledtransmission time. A related problem in battery operated systems islimitation of the transmitted power to conserve the battery power. Thefrequency hopping system, if designed properly, can be advantageouslyused to combat multipath fading that is a major source of transmittedsignal attenuation. Consequently, proper method and construction of thereceiver is of great importance. The system design and the receiverdesign should be done to support each other advantageously.

For example according to U.S. Pat. No. 5,428,637 granted to Oliva, etal., a method is described to reduce the synchronization overhead infrequency hopping systems to reduce the burden of resynchronizationbefore each separate transmission. The method is based on allocation ofspecific time slots for any unit that desires to transmit data and thusthe method requires a two-way communications to accomplish the necessaryexchange of series of reservation and acknowledge messages.

Similarly, in yet another example according to U.S. Pat. No. 5,438,329granted to Gastouniotis et al., a two-way system is used for efficientoperation of a telemetry system that is designed to allow operation inthe presence of multipath fading and interference.

Patent References:

    ______________________________________                                        Patent No.                                                                           Inventor  Issued  Title                                                ______________________________________                                        4843638                                                                              Walters   6/89    "Receiver for frequency                                                       hopped signals."                                     5428602                                                                              Kemppainen                                                                              6/95    "Frequency-hopping arrangement for                                            a radio communication system."                       4614945                                                                              Brunius   9/86    "Automatic/remote RF instrument                                               reading method and apparatus."                       5659303                                                                              Adair     8/97    "Method and apparatus for transmit-                                           ting monitor data."                                  5428637                                                                              Oliva     6/95    "Method for reducing synchronizing                                            overhead of frequency                                                         hopping communications systems."                     5438329                                                                              Gastouniotis                                                                            8/95    "Duplex bi-directional multi-mode                                             remote instrument reading                                                     and telemetry system."                               5408506                                                                              Mincher   4/95    "Distributed time synchronization                                             system and method."                                  4653068                                                                              Kadin     3/87    "Frequency hopping data                                                       communication system."                               4606041                                                                              Kadin     8/86    "Frequency hopping data                                                       communication system."                               5390166                                                                              Rohani    2/95    "Method for recovering a data signal                                          using diversity in a radio frequency,                                         time division multiple access                                                 communication system".                               5546422                                                                              Yokev     8/96    "Method for transmitting low-power                                            frequency hopped spread                                                       spectrum data."                                      5079768                                                                              Flammer   1/92    "Method for frequency sharing in                                              frequency hopping                                                             communications network."                             5121407                                                                              Partyka   6/92    "Spread Spectrum Communications                                               System."                                             ______________________________________                                    

Book References:

Robert Dixon, "Spread Spectrum Systems", John Wiley and Sons, 1884, ISBN0-471-88309-3.

Marvin K. Simon et al, "Spread Spectrum Communications, vol. 1,2,3",Computer Science Press, 1985, ISBN 0-88175-017-4.

Don J. Torrieri, "Principles of Secure Communication Systems", ArtechHouse, 1985, ISBN 0-89006-139-4.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a radiotransmission system including many radio transmitters that use frequencyhopping carrier to intermittently transmit very short messagesindicative of status of sensors associated with the transmitters, and toprovide a synchronization means and method that allows a frequencyhopping receiver to acquire and to maintain synchronizationsimultaneously with all the transmitters, thus, relieving transmittersfrom transmitting a long preamble each time a message is transmittedthat may otherwise be required if an acquisition is performed separatelyfor each message and which would result in an excessive current drainand shortened battery life in battery operated transmitters.

It is another object of this invention to provide a method oftransmission in such a system so as to improve reliability of the systemin the presence of multipath fading and interference.

It is a further object of this invention to provide a method ofeliminating the effect of persistent collisions in such a system thatoccur when two or more transmitters transmit at the same time and at thesame frequency for a prolonged period of time.

It is still a further object of this invention to provide a method thatallows such a system to convey the information about an abnormal sensorcondition as soon as the condition occurs regardless of the transmissionperiod of the associated transmitter.

It is still a further object of this invention to provide a method oftransmission resistant to a deliberate interference and having a meansto differentiate between valid and fraudulent transmissions.

According to one aspect of the invention there is provided a frequencyhopping radio transmission system comprising a plurality of transmittersand associated sensors and a receiver wherein said transmittersintermittently transmit very short messages indicative of status of thesensors associated with the transmitters, wherein (1) each transmitterincludes a time interval generator means to produce pulses controllingthe time interval between successive transmissions, a frequencysynthesizer-modulator means to generate a modulated radio frequencycarrier signal wherein the frequency of the carrier changes in responseto programming the synthesizer by digital data, a reference frequencyoscillator providing a frequency reference from which the synthesizerderives carrier frequencies and, preferably, from which the timeinterval generator derives its timing, a transmitter control logic meansactivated in response to pulses from the time interval generator or asensor signal indicating an abnormal condition, wherein when activated,the transmitter control logic activates and programs the synthesizer sothat the transmitter carrier frequency is changed according to apredetermined frequency hopping algorithm, provides digital dataindicative of the sensor status and preferably battery status, andmodulates the carrier with the provided data; (2) the receiver includesa frequency selective radio receiver circuit, programmable by digitaldata, to receive and demodulate a transmitted carrier when the frequencyof the receiver circuit is programmed according to the frequency of thecarrier, and a receiver control logic means to process demodulated data,to provide system interface responsive to the received data, and toprogram the frequency of the frequency selective receiver circuit,wherein the control logic includes a receiver timer to measure theelapsing time, and a plurality of memory registers to hold digital dataindicative of (a) the time of the next transmission occurrence for eachtransmitter and (b) the frequency of the next transmission occurrencefor each transmitter, wherein in operation, the control logicsequentially compares the data content of the time registers with thedata content of the timer and if the transmission is due from atransmitter, the control logic programs the frequency selective radioreceiver circuit according to the data content in the frequency registerassociated with said transmitter, attempts to decode the demodulatedsignal, modifies the content of the time register by a numberrepresentative of the time interval between the successive transmissionsfor said transmitter and modifies the content of the frequency registeraccording to a predetermined algorithm for said transmitter.

According to the second aspect of the invention, there is provided amethod of transmission in the system so as to improve reliability of thesystem in the presence of multipath fading and interference, the methodis based on arranging the frequencies available for transmission in aplurality of groups of frequencies, wherein each said group consists ofa predetermined number of frequencies selected in such a way that theyare approximately uniformly distributed in the entire available spectrumand separated by large but uneven frequency intervals, wherein a singlemessage is transmitted on one or more frequencies in one group andsubsequent messages are transmitted on the next frequencies in the groupuntil all frequencies in the group are used, then a new group isselected and subsequent messages are transmitted using the frequenciesfrom the new group and so on until all frequencies in all groups areused. Then, the process is repeated. Wherein the order in which thegroups and the frequencies in the groups are selected is determined inaccordance with a predetermined algorithm.

According to the third aspect of the invention, there is provided amethod of minimizing the effect of collisions, the method is based onselecting the sequence to use the frequencies within each group andselecting the sequence in which the groups are used to be different foreach transmitter, wherein resulting transmitter frequency sequencedepends on the transmitter ID number or other number which is includedin the transmitted message, so that, upon reception of a message from atransmitter, the receiver can determine what is the next frequency forthis transmitter.

According to the fourth aspect of this invention, there is providedanother method of minimizing the effect of collisions that can be usedalone or in conjunction with the third aspect of this invention, themethod comprising randomizing the time interval between transmissionsindividually for each transmitter and a receiver compensating for thetime interval changes.

According to the fifth aspect of this invention, there is provided amethod that allows such a system to convey the information about anabnormal sensor condition as soon as the condition occurs regardless ofthe transmission period of the associated transmitter. The methodcomprises of selecting an alarm frequency or preferably a group of alarmfrequencies common for all transmitters. The alarm frequencies are usedby the transmitters when an alarm or an abnormal sensor conditionoccurs, wherein when such a condition occurs in a transmitter, thetransmitter transmits the messages sequentially on the alarm frequenciesfor a predetermined period of time after which the transmitter resumestransmissions according to the sequence before the alarm condition,wherein the receiver monitors the alarm frequencies during the timebetween the reception of scheduled messages from the transmitters.

According to the sixth aspect of this invention there is provided amethod that allows the receiver to verify quickly whether the receivedmessage belongs to one of the transmitters associated with this receiveror some other spurious source without waiting for a complete messagetransmission. The method is based on encoding the transmitted pattern byinterleaving the transmitted data with a predetermined pattern that canbe decoded by the receiver without the reception of the entire message.Equivalently, the receiver can monitor other unique features of thereceived signal, for example modulation index or format, to accomplishthat.

These and other objects, advantages and features of this invention willbe apparent from the following detailed description of illustrativeembodiment that is to be read in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a transmitter according to a preferredembodiment of the present invention;

FIG. 2 is a block diagram of a receiver according to a preferredembodiment of the present invention;

FIG. 3a is a block diagram of a preferred implementation of a sequencegenerator used to address column of a frequency matrix;

FIG. 3b is a preferred implementation of the frequency matrix.

FIG. 4 is a block diagram depicting the frequency hopping systemincluding many transmitters and a receiver.

DETAILED DESCRIPTION

Referring to FIG. 4, the frequency hopping system includes a radioreceiver 401 and a plurality of radio transmitters 402, 403, 404 and405. The radio receiver includes a system interface 410 through whichthe receiver can be connected to a variety of interface equipment, acontroller, or a computer. Each transmitter includes a sensor or anoperation to be monitored. Each transmitter are not connected to eachother and do not receive messages back from the receiver. Thetransmitters transmits messages when they need to without any regard toother transmitters, as the transmitters are not synchronized with eachother.

Referring to FIG. 1, the transmitter includes a reference frequencycrystal oscillator 6 to produce a stable frequency on line 26, a timeinterval generator 2 establishing a time base to produce pulses on line28 activating the transmitter, a frequency synthesizer-modulator 4 toproduce a radio frequency carrier modulated by modulation data fed tothe synthesizer via line 24 wherein the frequency of the carrier isprogrammed to a desired value via plurality of lines 14, transmittercontrol logic 8 to activate and program the synthesizer-modulator 4 viaplurality of lines 14 when the logic is activated by a pulse from thetime interval generator or by an abnormal signal indication on a sensorsignal input line 18, an amplifier 10 to amplify the radio carrierprovided by the synthesizer when the amplifier is activated by thecontrol logic 8 via line 16, and an antenna 12 to radiate the powerdelivered by the amplifier. The control logic 8 includes a frequency andtime data memory register 20 to hold information used to determine thetime and the frequency of next transmission, and a sensor interfacecircuit 22 to accept the sensor signal and detect an abnormal signalcondition, and to convert the sensor signal to a digital format suitablefor transmission. The transmitter logic also includes a storage means 30to store a transmitter ID number to differentiate this transmitter fromother transmitters. The transmitter control logic, in some systems, canbe realized based on a microprocessor, in some other systems, aspecialized component may be used.

In operation, during the time between transmissions, the transmitter isin a standby mode in which the amplifier 10 and synthesizer-modulator 4are not active and, preferably, the control signals turn off the powerfrom these circuits in order to minimize the standby current of thetransmitter. The transmitter control logic 8 is in a standby mode inwhich the most of the circuits are inactive and some or most of thecircuitry can be powered down with the exception of the circuitssupporting critical finctions; (a) the sensor interface circuit 22 thatdetects an abnormal signal condition and produces a binary signal thatis logically combined with the signal 28 produced by the time intervalgenerator so that when either a pulse or abnormal condition occurs therest of the transmit logic circuitry is activated or powered up, (b) thefrequency and time data memory 20 that has to retain the data during theperiod between transmission and consequently either it has to be anonvolatile type or it has to be powered up during the period betweentransmissions. Upon activation, the control logic 8 determines theactivation source by reading signals 28 and 18.

When the logic 8 is activated by a pulse 28 from the time intervalgenerator the following sequence of events occurs. First, the logicreads the frequency data memory and produces a data packet that includesthe sensor status, the transmitter ID number and other data such asbattery status. Then, the logic activates and programs thesynthesizer-modulator 4, activates the amplifier 10 and sends the packetto the modulator via line 24. After completion of each transmission, thetransmitter logic sets the transmitter in the standby mode untilactivated again by a pulse on line 28 or a sensor abnormal conditionindicated on line 18.

In the preferred embodiment the transmission of a packet can be repeateda predetermined number of times at separate frequencies, wherein thenumber of repetitions is chosen according to application needs and,wherein the frequencies are determined by the transmitter logicaccording to an algorithm described later in details. This way it ispossible for the receiver to receive some repeated packets even if theother packets are lost due to frequency selective fading caused bymultipath or due to interference.

When a sensor abnormal condition occurs, the sensor interface circuit 22produces an active level of the signal indicative of the sensor abnormallevel which activates the transmitter via a combinatorial logic circuitthat combines the sensor abnormal level signal with the pulses from thetime interval generator. When activated this way, the transmittercontrol logic 8 produces a data packet that includes the sensor status,then the logic activates and programs the synthesizer-modulator 4,activates the amplifier 10, and sends the packet to thesynthesizer-modulator. In the preferred embodiment, the transmission ofthe alarm packet is repeated a predetermined number of times using aplurality of predetermined alarm frequencies in such a way that thetransmission frequency is changed after each single packet transmissionaccording to a predetermined fixed sequence. In the preferredimplementation, when the alarm packets are transmitted, the timeintervals between transmissions are minimal; when one transmission iscompleted, the transmitter immediately programs to the next frequencyand repeats the packet transmission, etc. In the preferred embodimentthere are eight frequencies used for this purpose, as described below.It should be noted that the existence of the predetermined alarmfrequencies is not necessary albeit advantageous. In an alternativedesign, the transmitter may follow the normal hopping pattern but anincreased rate repeating the alarm message a predetermined number oftimes. The essence of the idea is that the alarm message beinginfrequent can afford a much greater transmission overhead and can berepeated many times. If the alarm message is transmitted at fewerfrequencies, a faster response of the receiver will be observed onaverage.

After the transmission sequence is completed, the control logic disablesthe signal indicative of the sensor abnormal status so that an abnormalsensor status can not activate the control logic. Then, the controllogic puts the transmitter in the standby mode until activated by apulse from the time interval generator. When subsequently activated, thetransmitter control logic performs the usual transmission sequence butthe data packets include information that the sensor condition isabnormal if the condition persists. When the abnormal conditionsubsides, the signal indicative of an abnormal status is enabled so thata subsequent occurrence of an abnormal condition can activate the logicand trigger a new alarm transmission sequence; thus, normal operation isrestored.

In the preferred embodiment, the use of frequencies is determined asfollows. First, the entire available spectrum is divided into aplurality of channels. The number of channels depends on the availablespectrum and the receiver bandwidth. In the preferred embodiment, a 26MHz bandwidth is divided into 173 channels, each channel having 150 kHz.Then 8 channels are selected so that they are separated by large butuneven frequency intervals. Wherein, the large interval is defined hereas comparable to the coherence bandwith of the transmission channel. Thelarge separation improves probability that if one of the channels isfaded due to the multipath, the next channel is not faded. The unevenseparation ensures that a single harmonic interference does notinterfere with all channels. In the preferred embodiment, these channelsare reserved for the transmission of abnormal sensor status and will bereferred to as alarm frequencies. These frequencies are excluded fromuse if the sensor status is normal. In addition, channels on each sideand immediately adjacent to each alarm frequency are also excluded fromuse in order to minimize interference with the alarm frequencies by thetransmitters transmitting status messages. Thus, total number of 24frequencies is reserved. From the remaining 149 frequencies, 128frequencies are selected in an arbitrary but preferably non-uniform wayand assigned indexes from 0 to 127 in such a way that a smaller indexcorresponds to a lower frequency. Then the frequencies are organized ina matrix in the following way. There are 8 columns and 16 rows in thematrix. The frequencies in the first column are from f₀ to f₁₅, in thesecond column from f₁₆ to f₃₁, and so on. This way, the frequencies inthe first row are f₀, f₁₆, f₃₂, f₄₈, f₆₄, f₈₀, f₉₆, f₁₁₂, thefrequencies in the second row are f₁, f₁₇, f₃₃, f₄₉, f₆₅, f₈₁, f₉₇,f₁₁₃, and so on. Consequently, the frequencies in each row are separatedby large and uneven frequency intervals. Referring to FIG. 3b thefrequency matrix 300 has eight columns numbered 0 to 7 and 16 rowsnumbered 0 to 15. This way of organizing the matrix of frequencies isthe same for each transmitter and the receiver. However, in thepreferred embodiment, the sequence in which these frequencies are usedis different for different transmitters. The following is thedescription how this is accomplished in the preferred embodiment. Eachtransmitter includes two pseudo random sequence generators, wherein apseudo random sequence generator is based on a linear feedback shiftregister, wherein some outputs of the shift register are fed back to anEX-OR (Exclusive OR) gate whose output is connected to the registerinput. For a certain combination of the outputs that are fed to theEX-OR gate, the shift register can produce a sequence that has 2^(N) -1bits, wherein N is the length of the shift register. Such a sequence iscalled a maximum length sequence. Alternatively, if all the outputs ofthe shift register are taken at a time, then a pseudo random sequence of2^(N) -1 numbers is created, wherein all the numbers are N digits longand each number differs from all the other numbers in the sequence; thenumbers range from 1 to 2^(N) -1. Such pseudo random generators areknown to the skilled in the art and do not require additionaldescription. The first generator is based on a three-bit shift registerwith feedback taken from the first and the last bit. This registerproduces a sequence of seven numbers, wherein each number has threedigits. The numbers change from 1 to 7.

Referring to FIG. 3a, the pseudo random sequence generator 203 consistsof a shift register 205 and EX-OR gate 204. The shift register 205 iscomposed of three stages 221, 222, and 223 having three outputs Q₀ 211,Q₁ 212 and Q₂ 213 respectively. The feedback is taken from outputs Q₀and Q₂. The three least significant bits of the transmitter ID {t₀, t₁,t₂ } 201 are combined with the output of the pseudo random sequencegenerator {Q₀, Q₁, Q₂ } using EX-OR gates 206, 207, 208. The result isthe column address {C₀, C₁, C₂ } 202.

Assuming that the initial state of the shift register is binary 111(decimal 7), the produced sequence is {7, 3, 5, 2, 1, 4, 6}. Thesenumbers are then combined with the last three bits of the transmitter IDusing bit by bit EX-OR operation; i.e. the last bit of the transmitterID (t₀) is combined with the last bit of the random number (Q₀), etc.This way produced new sequence has numbers ranging from 0 to 7 the orderof which depends on the last three bits of the transmitter ID. Thus, 8distinct (permutated) sequences of numbers are created. These sequencesare used to address the columns of the frequency matrix. For example, ifthe last digits of the transmitter ID are 000, then the columns areselected in the order 7, 3, 5, 2, 1, 4, 6, i.e. the sequence is notaltered. If the last three digits of the transmitter ID are 001, thenthe columns are selected in the order 6, 2, 4, 3, 0, 5, 7; if the lastthree digits of the transmitter ID are 010, then the columns areselected in the order 5, 1, 7, 0, 3, 6, 4; etc.

In the preferred embodiment, the second pseudo random sequence generatorhas 4 bits; the feedback is taken from the first and the fourth bit. Theresulting four bit numbers are combined using bit by bit EX-OR operationwith the next 4 digits of the transmitter ID i.e. with the bits fourth,fifth, sixth and seventh to produce 16 sequences of numbers varying from0 to 15. The resulting new sequences are used to address the rows of thefrequency matrix. The column address is changed faster than the rowaddress; the column address is changed first until all seven numbers ofthe column address sequence are used, then the row address is changedthen the column address changes are repeated etc. It is apparent, thatany two sequences are quite different even though the ID number ischanged only on one position. This is advantageous since it lowers theprobability of persistent collision that may happen if two or moretransmitters transmit at the same time and at the same frequency for aprolonged time. Using the method of constructing hopping frequenciesdescribed above, 128 different sequences are created. This way theprobability of persistent collision is small even in systems with greatconcentration of transmitters. It should be stressed that using thesequences as described ensures that the persistent collision is notpossible since the frequencies in any arbitrary pair of sequences do notcoincide persistently regardless of the relative shift of the sequences.

It should now be also apparent, that this way or organizing and usingthe frequencies ensures that the successive transmissions from atransmitter will occur on frequencies that are always separated by largeand uneven frequency intervals. The large separation improvesprobability that if one of the channels is faded due to the multipath,the next channel is not faded. The uneven separation ensures that asingle harmonic interference does not interfere with all channels.

For each transmitter, the future frequency can be predicted based onjust one partially received message since each message includes thetransmitter ID based on which the receiver can determine the content ofthe row and column address generators for the current frequency. It isonly necessary to know the last 7 digits of the transmitter ID. Thesedigits are placed toward the end of the message in the preferredembodiment so that in case when the receiver begins reception of thetransmitted message starting in the middle of the message, thetransmitter ID can still be recovered and thus the next frequency can bepredicted.

In some applications, this number of sequences my not be sufficient. Insuch cases the number of sequences can be extended to a larger numberusing other techniques, some of which were extensively studied and aredescribed in the available literature. The number of available sequencesfor column selection is large and equals 8!=40320. Similarly the numberor possible sequences for row selections is 16! or approximately 2E13.However, the method described above is preferred for its simplicity andthe unique properties or orthogonality of all sequences. The degree oforthogonality indicates how many hits (frequency agreements) there maybe between two sequences upon any relative cyclic shift of thesequences. In a perfect design, for any two sequence that use the sameset of frequencies, there would be only one hit. I.e. if upon any cyclicshift of two sequences, a position is found in which the same frequencyis present in both sequences, then the frequencies in all otherpositions would differ. The sequences produced in a manner as describedin the preferred embodiment are orthogonal in that sense. Althoughperfect orthogonality is not necessary for proper operation of thesystem, it is desirable since it reduces the probability of lost packetsdue to collisions. However, it should be apparent that other ways ofarranging the frequencies and using this method of randomizing could becreated.

Normally, the time intervals between transmissions are controlled by aquartz crystal and, ideally their nominal values are the same for alltransmitters, however in the preferred embodiment, the time intervalsare perturbed by predetermined small time increments delta T to furtherrandomize the transmission events and lower the probability ofpersistent collisions with other transmitters as well as avoiding anintentional or unintentional pulsed interference. The transmittercontrol logic can accomplish this by programming the time intervalgenerator via line 26 according to a predetermined algorithm. Theinformation about the current status of the algorithm may be included inthe transmitted packet to aid the receiver operation.

In the preferred embodiment, the method of determining the time intervalperturbation is based on similar technique as described in conjunctionwith row address generation for the frequency matrix, wherein the randomsequence is used to alter the time interval between transmissions. I.e.each time a transmission is performed, a new number is generated andused to determine the time interval between the current and the nexttransmission. Wherein, the time randomization is accomplished bycombining the output of the three-bit pseudo random generator used forcolumn addressing in the frequency matrix with the bits eight, nine andten of the transmitter ID. I.e. bit Q₀ is combined with bit t₇, bit Q₁with bit t₈, etc. The resulting 3-bit numbers (max ±3) are used todetermine how many delta T increments are added to the predeterminednominal value of the time between transmissions to determine the time ofthe next transmission. This way, an instant synchronization is possible,including the time perturbation, based on a single received messagebecause the receiver can infer the status of the 3-bit generator basedon the received frequency index and the transmitter ID number. I.e. themessage contains the information about the 3-bit generator withoutexplicit inclusion of the generator status bits in the message.

It is to be understood that the random frequency selection as describedabove and the time perturbation can be used together or in separation toachieve immunity to collisions. I.e. (a) a fixed frequency pattern forall transmitters and random time perturbation patterns individual foreach transmitter can be used, or (b) a fixed time interval betweentransmission or fixed time perturbation pattern and random frequencyselection individual for each transmitter can be used, or (c) frequencyand time changes can be combined to enhance the system performance atthe expense of complication.

In the preferred embodiment, both the transmission frequency and thetime interval between transmissions are individually randomized for eachtransmitter.

According to another aspect of the invention, the receiver can examinethe transmitted waveform looking for specific parameters such asfrequency deviation, modulation type, data encoding, or specific datapatterns included in the transmitted data. This is done by the receiverto differentiate between the signals from the transmitters that belongto the system and some spurious transmitters or interference. In onepreferred embodiment, the transmitter interleaves the data in thetransmitted packet with a predetermined pattern. The preferred patternis repeated 01 pattern resulting in the interleaved data:

    . . . b.sub.n-1 0 b.sub.n 1 b.sub.n+1 0 . . .

wherein b_(i) represents the original data bits before interleaving. Inmost cases, the differentiation can be done without waiting for thereception of an entire packet. If the interleaving pattern is notpresent, the receiver may quickly hop to examine a new frequency. Thisaids a faster and more reliable acquisition process during which thereceiver synchronizes its time and frequency coordinates of atransmitter with which the synchronization has been lost.

Referring to FIG. 2, the receiver includes a reference frequency crystaloscillator 126 to produce a stable reference frequency on line 128 forthe receiver circuits, a frequency selective radio receiver circuit 100whose frequency is programmable via lines 116, to receive and demodulatea frequency modulated carrier when the frequency of the frequencyselective receiver circuit is programmed according to the frequency ofthe carrier, and a receiver control logic means 130 to processdemodulated data, to provide system interface lines 140, responsive tothe received data, and to program the frequency of the frequencyselective receiver circuit. The control logic includes a receiver timer132 establishing a time base to measure the elapsing time. The controllogic also includes: (a) a plurality of ID memory registers 134 to holddigital data indicative of ID numbers for each transmitter that belongsto the system, (b) a plurality of time memory registers 136 to holddigital data indicative of the time of the next transmission occurrencefor each respective transmitter, and (c) a plurality of frequency memoryregisters 138 to hold digital data indicative of the frequency of thenext transmission occurrence for each respective transmitter. In thepreferred embodiment, the registers are organized such that an arbitraryregister i 151 of the plurality of ID memory registers 134 associatedwith a transmitter whose ID number is n, is associated with register i152 of the plurality of time memory registers 136 and register i 153 ofplurality of frequency memory registers 138, wherein said registers 152and 153 hold data associated with said transmitter n. The frequencyselective radio receiver circuit 100 includes a RF band pass filter 104,an amplifier 106, an IF bandpass filter 110, a mixer 108,limiter-discriminator circuit 112 and frequency synthesizer 114. The RFband-pass filter selects only the desired frequency band allocated forthe transmission, the mixer mixes the incoming signal with the signalproduced in the frequency synthesizer and produces an IF frequency(Intermediate Frequency). The IF frequency is filtered in a narrow bandfilter 110 whose bandwidth is selected according to the channelbandwidth. The limiter discriminator demodulates the signal and producesbaseband DATA signal 120 and an RSSI signal 118 indicative of thereceived signal strength. The DATA signal 120 and the RSSI signal 118are converted to binary signals by A/D converters 124 and 122respectively and fed to the control logic 130. The presentedarchitecture of the frequency selective radio receiver circuit 100 isknown as a superheterodyne FM receiver, it is very well known and itdoes not require additional explanation. The transmitted message data isextracted from the DATA signal 120 digitized by the A/D converter 124using one of the many well-known methods for signal processing and doesnot require additional explanation.

In the preferred embodiment, the frequency registers 138 hold for eachtransmitter the state of the row and column address generators foraddressing the frequency array. The frequency array is as describedabove and it is identical to the arrays in all transmitters. If thesynchronization is obtained with a given transmitter, the states of therow and column address generators are identical with that in thetransmitter. In the preferred implementation, the time registers 136hold numbers--time of next transmission--for each transmitterrepresenting the state of the receiver timer 132 at the time the nexttransmission is due from a transmitter.

In operation, the receiver control logic 130 sequentially compares thedata content of the time registers 136 with the data content of thereceiver timer 132 and if the transmission is due from a transmitter,the control logic programs the frequency selective radio receivercircuit 100 according to the data content in the frequency register 138for this transmitter, attempts to decode the demodulated signal, changesthe content of the time register based on the number representative ofthe time interval between the transmissions for this transmitter andchanges the content of the frequency register according to apredetermined algorithm for this transmitter. I.e. the frequency and thetime registers are updated each time a transmission is due regardlesswhether the packet was received successfully. The new content of thefrequency register is determined according to the algorithm for thefrequency use by the transmitters.

The new content of the time register is calculated based on the currentcontent of the receiver timer and a number representative of the timebetween the current transmission and the next transmission for thistransmitter, wherein said number is calculated based on the nominalvalue of the time between the transmissions and adjusted by the pseudorandom perturbation performed according to the previously describedalgorithm. In addition, said number is corrected by a correction factorbased on the measured difference between the transmitter time base andthe time base of the receiver, wherein said difference is determined ina manner described later in details. In the preferred embodiment, thenumbers representative of the time base differences are stored in thetime registers 136 separately for each transmitter and are independentfrom the numbers representing the time of the next transmission, i.e.the time registers are split to hold two independent numbers.

It should be noted that even if crystal oscillators are used in thetransmitters and the receiver to control the timing, the erroraccumulated during the time between transmissions may be significantcompared to the packet time. For example, if the period between thetransmissions is 100 seconds and the crystal frequency error due totolerance and temperature changes is ±20 ppm (parts per million) for thetransmitter and ±10 ppm for the receiver, then the error may be as largeas 3 ms. If the time for the transmission of one packet is 5 ms, thenthe error is significant. In order to minimize the time erroraccumulated during the long time between the transmissions, the receivercan store the time difference between the ideal and the actual time ofthe packet reception and use the difference to predict more accuratelythe next transmission time. For example, if the timer resolution is 0.3ms, then the next transmission time can be predicted with accuracy 0.3ms, providing that the temperature does not change appreciably over 100s period. This represents an improvement of an order of magnitude. I.e.the receiver can program its frequency 0.3 ms in advance to each newfrequency, examine it for the duration of the packet, then program tothe next frequency and so on.

During the acquisition, when the time error is not known, the receiverneeds to tune to the first frequency at least 3 ms in advance. Then, thereceiver monitors the received signal by observing the RSSI signal 118and DATA signal 120. If during the next 6 ms no valid signal is present,the receiver programs to the next frequency 3 ms in advance and so on.To avoid this acquisition problem, the receiver can include a frequencyerror detection means as described below.

In the preferred embodiment, the receiver includes a frequency errordetection means 142 that is preferably implemented as a simple digitalcounter, in order to detect the frequency error in the received signalin respect to the receiver reference frequency by measuring thefrequency error of the intermediate frequency signal 111. In addition,in the transmitter, the transmitted carrier frequency and the timeinterval generator timing are derived from the same source and in thereceiver, the receiver frequency and the receiver timer are derived fromthe same reference. In operation, the receiver can measure the frequencydifference between the transmitted carrier and the receiver frequencyand use the measured error to determine the difference between thetransmitter reference frequency and the receiver reference frequencybased on just one partially decoded message. The frequency differencemeasurement is accomplished in the following way. Assuming that thetransmitter frequency accuracy is ±20 ppm and the receiver is ±10 ppm,carrier frequency is 915 MHz and IF frequency is 10.7 MHz, the absolutemaximum error between the receiver frequency synthesizer and thereceived carrier can be as much as 2760 Hz (915E6*20E-6+925.7E6*10E-6).I.e. the resulting IF frequency is offset from its nominal value by thisamount. This represents 260 ppm of the nominal IF frequency. An ordinaryfrequency counter with a time base accuracy determined by the receivercrystal oscillator, i.e. ±10 ppm can detect this error and measure itwith good accuracy. The accuracy should be better than ±110 Hz (1/26 ofthe maximum error). Based on the measured frequency error, the relativefrequency offset is calculated and the time correction factor for eachtransmitter is adjusted accordingly. For example, if the measured erroris +1380 Hz then the relative frequency error is approximately equal to+15 ppm. If the nominal value of the time interval between twoconsecutive transmissions is 100 seconds, then the required correctionis +1.5 ms if the receiver uses high injection, i.e. the frequency ofthe synthesizer in the receiver is nominally equal to the receivedfrequency plus the IF frequency, and -1.5 ms if low injection is used.

In the preferred embodiment, the time base correction factor stored foreach transmitter is also used to adjust the center frequency of thereceiver and thus aid the reception of the transmitted packets, thuslowering the requirements for the length of the preamble included ineach packet for the purpose of carrier and data timing acquisition. Thisis accomplished by adjusting the receiver frequency momentarily justprior to the reception of the packet from a transmitter from which apacket is due.

In operation, the receiver scans the alarm frequencies during the timewhen it is not occupied with the scheduled reception from thetransmitters or checking the time registers. Also the receiver scans allthe available frequencies in addition to the alarm frequencies. Duringthe scan, the receiver uses RSSI signal to detect if there is an energytransmitted on a current frequency; if so then the receiver determinesif the transmitted pattern is valid by detecting the interleaved patternin the message or other unique properties of the modulated carrier. Ifthe energy is not present or the pattern is not valid, the receiver willquickly proceed to examine the next frequency. Only if the pattern isvalid, the receiver will stay on this frequency and try to decode themessage. This way all the alarm frequencies are examined several timesper second ensuring that the receiver can receive the alarm message witha minimum delay. Also, the scan of all available frequencies is fast;the synchronization can be regained faster and more reliably because thereceiver will not waste much of the time for an examination of very weakor spurious signals.

In the preferred embodiment, when a transmitter is powered up, forexample after a battery replacement, it enters a power-up mode duringwhich a predetermined number of packets are transmitted on the alarmfrequencies in a way similar to the transmission of alarm packets. Inthe power-up transmission sequence, each packet includes a number thatindicates how many packets the transmitter will transmit in this modebefore entering a normal mode of operation. This way, the receiver cansynchronize with the transmitter just after a single packet reception bycalculating when the first transmission will occur in the normal mode.

In the preferred embodiment, the transmitter ID numbers for eachtransmitter stored in the receiver ID memory registers 134 are acquiredand stored by the receiver during a process of log-in. Each newtransmitter to be logged-in is placed in a close proximity to thereceiver and then powered up. A very high level of the received signalensures that the new transmitter signal is not mistaken for anothertransmitter. A successful log-in is confirmed by the receiver using anaudio or a visual indicator that can be included in the receiver or inthe system controller connected to the receiver via system interface140. The receiver may reject the transmitters that can cause persistentcollisions i.e. if its ID number has the last 10 digits identical toanother transmitter already present in the system.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in art without departing from the scope andspirit of the invention as defined by the appended claims.

What is claimed is:
 1. A radio transmission system comprising of a radioreceiver and a plurality of radio transmitters using frequency hoppingmodulation to intermittently transmit messages indicative of a status ofoperation associated with the transmitters;wherein each said transmitterincludes a control logic means to change the carrier frequency accordingto a predetermined pattern and a time interval generator means foractivating said transmitter at predetermined time intervals, saidintervals may be set at a predetermined nominal value or changedaccording to a predetermined pattern; and wherein said receiver includesa frequency selective radio circuit that can be programmed to a desiredfrequency to receive and demodulate the transmitted carriers; andwherein said receiver includes a plurality of digital counters or memoryregisters means to hold data indicative of (a) the time of the nexttransmission occurrence for each said transmitter and (b) the frequencyof the next transmission occurrence for each said transmitter, andwherein said receiver includes a receiver control logic means to changethe receiver frequency according to said data in said digital countersor memory registers and to change said data in said digital counters ormemory registers in response to the received messages; and wherein, saidreceiver control logic is operative to change the data in said digitalcounters or memory registers based on the time of arrival, the carrierfrequency and the content of a message received from a transmitter forsubsequent use for the determination of the time and frequency of thefuture transmissions for said transmitter, said determination is madeindividually for each transmitter of said plurality of transmitters,thus allowing the receiver to receive messages from many unsynchronizedfrequency hopping transmitters.
 2. A frequency hopping radiotransmission system according to claim 1, wherein said frequencypatterns in said transmitters are predetermined in such a way that thesuccessive frequencies are arranged in groups, said groups havingpredetermined number of frequencies chosen in such way that saidfrequencies in each said group are distributed over the entire availablebandwidth and are separated by large but preferably uneven frequencyintervals.
 3. A frequency hopping radio transmission system according toclaim 1, wherein said frequency patterns are predetermined individuallyfor each transmitter.
 4. A frequency hopping radio transmission systemaccording to claim 3, wherein said frequency patterns are predeterminedindividually for each transmitter based on a transmitter ID number, saidnumber or a number derived from it being included in the transmittedmessages, and wherein said frequency patterns are obtained by a methodcomprising:arranging the available frequencies in a matrix, wherein saidmatrix have a predetermined number N_(R) of rows and a predeterminednumber N_(C) of columns, wherein the frequencies in the rows aredistributed over the entire available bandwidth and separated by largeand preferably uneven frequency intervals; producing a first pseudorandom number in a first pseudo random generator having a predeterminednumber G_(C) of bits and encode said first number with a predeterminednumber of bits of the transmitter ID by processing said generator bitswith said transmitter ID according to a predetermined algorithm toproduce column address for said matrix; producing a second pseudo randomnumber in a second pseudo random generator having a predetermined numberof G_(R) bits and encoding said second number with a predeterminednumber of bits of the transmitter ID by processing said generator bitswith said transmitter ID according to a predetermined algorithm;changing the column address until all possible addresses are selected,then changing the row address to the next value, then repeating thesequence of column address changes etc.
 5. A frequency hopping radiotransmission system according to claims 1 or 3 wherein said timeintervals are changed according to predetermined patterns.
 6. Afrequency hopping radio transmission system according to claim 3,wherein an information is included by each said transmitter in thetransmitted messages that identifies which patterns are used by saidtransmitter.
 7. A frequency hopping radio transmission system accordingto claim 6, wherein said individual determination for each transmitteris based on a transmitter ID or another number derived from orassociated with said ID.
 8. A frequency hopping radio transmissionsystem according to claim 7, wherein said predetermined patterns arebased on identical pattern generators in each transmitter, Saidindividual determination is obtained by modifying the output of saidpattern generator in each transmitter.
 9. A frequency hopping radiotransmission system according to claim 5, wherein an information isincluded by each said transmitter in the transmitted messages thatidentifies which patterns are used by said transmitter.
 10. A frequencyhopping radio transmission system according to claim 9, wherein saidindividual determination for each transmitter is based on a transmitterID or another number derived from or associated with said ID.
 11. Afrequency hopping radio transmission system according to claim 10,wherein said predetermined patterns are based on identical patterngenerators in each transmitter, Said individual determination isobtained by modifying the output of said pattern generator in eachtransmitter.
 12. A frequency hopping radio transmission system accordingto claim 5, wherein information about the current status of said timeinterval pattern is preferably contained in the transmitted messages.13. A frequency hopping radio transmission system according to claim 5,wherein said time interval pattern and said frequency pattern in atransmitter are based on a same pattern generator and said frequencypattern is obtained by a first modification of the generated pattern andsaid time interval pattern is obtained by a second modification of thegenerated pattern.
 14. A frequency hopping radio transmission systemaccording to claim 5, wherein said patterns are predeterminedindividually for each transmitter.
 15. A frequency hopping radiotransmission system according to claim 14, wherein an information isincluded by each said transmitter in the transmitted messages thatidentifies which patterns are used by said transmitter.
 16. A frequencyhopping radio transmission system according to claim 15, wherein saidindividual determination for each transmitter is based on a transmitterID or another number derived from or associated with said ID.
 17. Afrequency hopping radio transmission system according to claim 16,wherein said predetermined patterns are based on identical patterngenerators in each transmitter, Said individual determination isobtained by modifying the output of said pattern generator in eachtransmitter.
 18. A frequency hopping radio transmission system accordingto claim 14, wherein information about the current status of said timeinterval pattern is preferably contained in the transmitted messages.19. A frequency hopping radio transmission system according to claim 14,wherein said time interval pattern and said frequency pattern in atransmitter are based on a same pattern generator and said frequencypattern is obtained by a first modification of the generated pattern andsaid time interval pattern is obtained by a second modification of thegenerated pattern.
 20. A frequency hopping radio transmission systemaccording to claim 1, wherein messages requiring immediate attention arerepeatedly transmitted by a transmitter on one or more frequencies for apredetermined time duration or a predetermined number of timesimmediately upon occurrence of an extraordinary condition requiringimmediate attention, regardless of the status of the time intervalgenerator of said transmitter.
 21. A frequency hopping radiotransmission system according to claim 20, wherein when powered-up, saidtransmitter repeatedly transmits a predetermined message on one or moreof said alarm frequencies for a predetermined time duration orpredetermined number of times.
 22. A frequency hopping radiotransmission system according to claim 21, wherein each transmittedmessage in the power-up mode includes a number indicative of the totalnumber of transmissions remaining before the transmitter ends thepower-up transmissions and enters a normal mode of operation.
 23. Afrequency hopping radio transmission system according to claim 20,wherein a new transmitter is added to the system using a methodcomprising:activating or powering up the transmitter to be added to thesystem, wherein said transmitter upon activation or powering up sendsmessages as described in claim 6; and receiver detecting said messagesand responding by storing said transmitter ID number in a memoryregister and by allocating time and frequency memory registers for saidtransmitter for subsequent use during operation, wherein the receiver isaided to obtain protection from a spurious log-in by bringing thetransmitter to a close proximity to the receiver and the receiver usingthe high signal strength of the received carrier as a basis todifferentiate valid new transmitter messages from spurioustransmissions.
 24. A frequency hopping radio transmission systemaccording to claim 1, wherein the transmitters are selected to operatein the system based on the differences in their ID numbers.
 25. Afrequency hopping radio transmission system according to claim 1,wherein said receiver detects and measures predetermined uniqueproperties of the transmitted signal, and said receiver uses the resultof the measurement to quickly differentiate between the signals from thetransmitters that belong to said system and other signals andinterferences without having to decode the entire message.
 26. Afrequency hopping radio transmission system according to claim 1,wherein the relative timing error between the time interval generator ina transmitter and the timing in said receiver is detected andcompensated in the receiver by measuring the difference between thenominal and actual time between transmissions for a transmitter andusing said difference to compute the next transmission time for saidtransmitter; wherein said detection and computation is done individuallyfor each transmitter.
 27. A frequency hopping radio transmission systemaccording to claim 1, wherein the time interval generator timing andtransmitted carrier frequency in said transmitters is derived from asame crystal oscillator in the transmitter; andwherein the receiverfrequency and time interval generator timing are derived from a samecrystal oscillator in the receiver; and wherein said receiver includes afrequency counter means to detect relative frequency error between thereceived carrier frequency and the receiver frequency; and wherein saidreceiver uses said measured frequency error to compute the timing errorin the time interval generator in said transmitter and to moreaccurately determine the time of the next transmission from saidtransmitter, wherein such determination is done individually for eachtransmitter, and wherein preferably the measured frequency error is alsoused to adjust the receiver center frequency to aid the reception of thetransmitted data individually for each transmitter.
 28. A frequencyhopping radio transmission system according to claim 1, wherein apredetermined number of alarm frequencies is assigned for the purpose oftransmitting messages requiring immediate attention, said messages arerepeatedly transmitted by a transmitter on one or more of thefrequencies for a predetermined time duration or a predetermined numberof times immediately upon occurrence of an extraordinary conditionrequiring immediate attention, regardless of the status of the timeinterval generator of said transmitter.
 29. An apparatus for receivingdigital data from a plurality of frequency hopping transmitterscomprising a receiver;said receiver including a frequency selectiveradio circuit that can be programmed to a desired frequency to receiveand demodulate the transmitted carriers; and said receiver having aplurality of (a) time registers for holding data indicative of the nexttransmission occurrence separately for each said transmitter and (b)frequency registers for holding data indicative of the frequency of thenext transmission occurrence separately for each said transmitter; andsaid receiver having a timer to measure the elapsing time; and saidreceiver using said receiver timer and said time and frequency registersto control said receiver frequency in the steps comprising: (1) saidreceiver determining which transmitter is the next transmitter totransmit a message on basis of the content of said time registers andchanging said receiver frequency to the next frequency of said nexttransmitter at such time in advance before said message is transmittedas to allow for the reception of entire said message, wherein saidfrequency is determined on basis of the content of said frequencyregister associated with said next transmitter; and (2) said receiver,upon complete or partial reception of a message from a transmitter,changing the data in said frequency register associated with saidtransmitter to indicate the next frequency for said transmitteraccording to a predetermined algorithm, and changing current content ofsaid time register associated with said transmitter, wherein said timeregister change is based on the time of the arrival of the receivedmessage and a number representative of the time interval between thecurrent and the next transmission for said transmitter according to apredetermined algorithm; and wherein (3) when said receiver fails todetect the transmitted signal at the due time due to signal fading orinterference, said receiver changes the data in said frequency registerassociated with said transmitter to indicate the next frequency for saidtransmitter according to a predetermined algorithm, and said receiverchanges the data in said time register associated with said transmitter,said time register change is based on the current status of said timeregister associated with said transmitter and the time interval betweenthe presently due transmission and the next transmission determinedaccording with a predetermined algorithm; and (4) said receiver usingthe available time between transmissions to sequentially scan otherfrequencies of a plurality of frequencies available for transmission inan attempt to receive signals from transmitters with whichsynchronization has been lost, and wherein when such a signal isdetected, updating said time register for said transmitter according tostep (2) and thus restoring synchronization; and (5) said receiverrepeating steps 1 through
 5. 30. An apparatus according to claim 29,wherein said receiver has a means to replicate the frequency hoppingpatterns individually for each said transmitter and said patterns aredifferent for each said transmitter and wherein said replication isbased on the data content in the received messages that identifies whichpattern is used by said transmitter from which said data is received.31. An apparatus according to claim 30, wherein said replicatedfrequency patterns are predetermined individually for each transmitterbased on the transmitter ID number, said number or a number derived fromit being included in the transmitted messages, and wherein frequencyhopping patterns are determined by a method comprising:arranging theavailable frequencies in a matrix, wherein said matrix has apredetermined number N_(R) of rows and a predetermined number N_(C) ofcolumns, wherein the frequencies in the rows are distributed over theentire available bandwidth and separated by large and preferably unevenfrequency intervals; producing a first pseudo random number in a firstpseudo random generator having a predetermined number G_(C) of bits andencode said first number with a predetermined number of bits of thetransmitter ID by processing said generated bits with said transmitterID according to a predetermined algorithm to produce column address forsaid matrix; producing a second pseudo random number in a second pseudorandom generator having a predetermined number of G_(R) bits andencoding said second number with a predetermined number of bits of thetransmitter ID by processing said generated bits with said transmitterID according to a predetermined algorithm; changing the column addressuntil all possible addresses are selected, then changing the row addressto the next value, repeating a sequence of column address changes forthe row, repeating column address changes for each row address, andwherein the status of said first and second pseudo random generators foreach said transmitter is kept in said frequency memory registersindividually for each transmitter and adjusted individually for eachtransmitter.
 32. An apparatus according to claim 29, wherein the timeintervals between transmissions are determined individually for eachtransmitter and are changed in a pattern that is different for eachtransmitter.
 33. An apparatus according to claim 32, wherein thereceiver makes use of the information contained in the messages receivedfrom a transmitter about which pattern is used by said transmitter. 34.An apparatus according to claim 29, wherein said receiver is using theavailable time between transmissions to scan for alarm messagesrequiring immediate attention that may be repeatedly transmitted bytransmitters on one or more frequencies according to a predeterminedalgorithm.
 35. An apparatus according to claim 34, wherein said scanoccurs over a predetermined number of alarm frequencies assignedspecially for the purpose of transmitting alarm messages.
 36. Anapparatus according to claim 29, wherein when a receiver detects amessage from a transmitter indicating that the transmitter is inpower-up mode, said receiver retrieves a number contained in the messageindicating how long the transmitter will be in that mode, and saidreceiver uses said number to determine the time of occurrence of thenext normal transmission for said transmitter after the power-upsequence is finished.
 37. An apparatus according to claim 29, whereinsaid receiver has a means to determine the signal strength of thereceived carrier, and wherein said receiver upon detection of a power-upsequence above a predetermined signal level from a new transmitter whoseID is not yet contained in the receiver memory responds by storing thenew ID, assigning a time memory register and frequency memory register,synchronizing with said new transmitter and subsequent monitoring ofsaid transmitter.
 38. An apparatus according to claim 37, wherein thestatus of adding said new transmitter is indicated by the receiver usingan audio or visual indicator or electrical signals on receiver interfacelines.
 39. An apparatus according to claim 29, wherein during theprocess of adding a new transmitter to the system, the receiver canreject a new transmitter whose certain digits of the ID number areidentical to another transmitter already present in the system.
 40. Anapparatus according to claim 29, wherein said receiver detects andmeasures predetermined unique properties of the transmitted signal, andsaid receiver uses the result of the measurement to quicklydifferentiate between the signals from the transmitters that belong tosaid system and other signals and interferences without having to decodethe entire message.
 41. An apparatus according to claim 29, wherein thetiming error between the nominal time between transmissions and theactual time between transmissions for a transmitter is measured by thereceiver and used to compute the next transmission time for saidtransmitter, wherein said measurement and computation is doneindividually for each transmitter.
 42. An apparatus according to claim29 to be used in a system in which the timer time and transmittedcarrier frequency in said transmitters are derived from a same crystaloscillator in the transmitter, wherein the receiver frequency and timertime are derived from a same crystal oscillator in the receiver;andwherein said receiver includes a frequency counter means to detectrelative frequency error between the received carrier frequency and thereceiver frequency; and wherein said receiver uses said measuredfrequency error to compute the timing error in the time intervalgenerator in said transmitter and to more accurately determine the timeof the next transmission from said transmitter, wherein suchdetermination is done individually for each transmitter, and whereinpreferably the measured frequency error is also used to adjust thereceiver center frequency to aid the reception of the transmitted dataindividually for each transmitter.
 43. An apparatus according to claim29, wherein a valid message detection on a wrong frequency or at a wrongtime triggers an alarm indicating detection of an imposter transmitterattempting to send a fraudulent message to the system.
 44. An apparatusaccording to claim 29, wherein a detection of jamming of more than apredetermined number of frequencies or a predetermined change in thenumber of jammed frequencies triggers an alarm.
 45. An apparatusaccording to claim 29, wherein a loss of a predetermined number ofpackets from a single transmitter triggers an alarm.